Sepsis is a medical syndrome characterized by an overwhelming systemic response to infection that can rapidly lead to shock, organ failure and death. Sepsis may also lead to the development of adult respiratory distress syndrome (ARDS), a life-threatening condition in which inflammation of the lungs and accumulation of fluid in the air sacs (alveoli) leads to low blood oxygen levels. According to data presented at the 67th annual scientific meeting of the American College of Chest Physicians in Philadelphia, the incidence of sepsis in the United States increased by 23.3% from 1988 to 1998. In the U.S., sepsis is the 10th leading cause of death overall, accounting for over 750,000 cases and 215,000 deaths each year and $17 billion in annual health care expenditures. Moreover, the incidence of sepsis may be rising due to the increasing age of the population, growing numbers of immunocompromised patients, use of life-sustaining technologies, and increased resistance of bacteria to antimicrobial agents.
Sepsis may be caused by bacterial (either Gram-positive or Gram-negative), fungal, viral, and other infections. Although sepsis can follow any bacterial infection, it is often associated with a Gram-negative bacterial infection. It is generally accepted that approximately 50% of sepsis cases due to bacterial infections are caused by Gram-negative bacteria. Most of the damage associated with Gram-negative sepsis comes not from the invasion of bacteria per se but from the endotoxin present in the cell wall of the bacteria. Endotoxin or lipopolysaccharide (LPS) is a toxin released from Gram-negative bacteria. Following its release into the blood stream from a site of infection, such as the lung, abdomen, or urinary tract, endotoxin acts on a number of different cell types, and induces a complex cascade of cellular, mediator and cytokine-related events. This inflammatory cascade results in organ (e.g., lung and kidney) damage, shock and death in patients with Gram-negative septicemia (endotoxemia). Based on the current understanding of how endotoxin induces this complex cascade of events, specific therapies developed in the past or currently in development attempt to target specific events in this cascade.
Endotoxin is released during growth, death and lysis of Gram-negative bacteria. See, for example, Rietschel et al. (1994) FASEB J 8:217-225; Hurley (1995) Clin. Microbiol. Rev. 8:268-292; and Mayeux (1997) J Tox. Environ. Health 51:415-435. Following its release into the circulation, endotoxin activates complement, coagulation, and kinin cascades. Endotoxin binds with an acute phase serum protein, LPS binding protein (LBP), and soluble CD14 (sCD14) to form LPS-LBP and LPS-sCD14 complexes. See Mayeux (1997) J Tox. Environ. Health 51:415-435; Chen et al. (1992) Curr. Topics Microbiol. and Immunol. 181:169-188; and Pugin et al. (1993) Proc. Natl. Acad. Sci. 90:2744-2748. These complexes then bind to specific binding proteins or specific cell membrane receptors on a number of different cell types, including vascular endothelial cells, neutrophils, and macrophages to induce the release of oxygen free radicals, nitric oxide, metabolites of arachidonic acid, including thromboxane (TXA2), prostacyclin, and platelet activating factor (PAF), and cytokines including tumor necrosis factor-α (TNF-α), interleukin-1 (IL-1), and IL-6. See Mayeux (1997) J Tox. Environ. Health 51:415-435; Chen et al. (1992) Curr. Topics Microbiol. and Immunol. 181:169-188; Pugin et al. (1993) Proc. Natl. Acad. Sci. 90:2744-2748; Akarasereenont et al. (1995) Eur. J Pharmacol. 273: 121-128; Brigham and Meyrick (1986) Am. Rev. Respir. Dis. 133:913-927; Morrison (1987) Ann. Rev. Med. 38:417-432; and Schletter et al. (1995) Arch. Microbiol. 164:383-389. These mediators and cytokines exert direct cytotoxic effects, promote neutrophil activation and migration, promote upregulation of adhesion molecules for neutrophils on endothelial cells, prime macrophages to the effects of endotoxin-induced cytokine release, and produce cardiovascular effects, including myocardial depression, vascular relaxation, and pulmonary hypertension that are important in the pathophysiology of endotoxin-induced organ injury and cardiovascular collapse. See generally See Mayeux (1997) J Tox. Environ. Health 51:415-435; Chen et al. (1992) Curr. Topics Microbiol. and Immunol. 181:169-188; Pugin et al. (1993) Proc. Natl. Acad. Sci. 90:2744-2748; Akarasereenont et al. (1995) Eur. J Pharmacol. 273: 121-128; Brigham and Meyrick (1986) Am. Rev. Respir. Dis. 133:913-927; Morrison (1987) Ann. Rev. Med. 38:417-432; Schletter et al. (1995) Arch. Microbiol. 164:383-389; Hoshino et al. (1999) J Immunol. 162: 3749-3752; Poltorak et al. (1998) Science 282: 2085-2088; and Martin and Silverman (1992) Clin. Infect. Dis. 14:1213-1228.
Endotoxin levels are increased in patients at risk of adult respiratory distress syndrome (ARDS), in patients with Gram-negative septicemia and ARDS, and in patients with septic shock and multisystem organ failure. See Parsons et al. (1989) Am. Rev. Respir. Dis. 140:294-301; Brandtzaeg et al. (1989) J Infect. Dis. 159:195-204; and Danner et al. (1991) Chest 99:169-175. There is a growing body of scientific information supporting the concept that endotoxin initiates this complex “sepsis cascade” of events that results in organ damage and septic shock by binding to a receptor on endothelial cells to induce the release of cytotoxic substances which produce early endothelial cell damage. In the lung, endotoxin causes structural changes in the microvasculature, resulting in disruption of the blood-air barrier, interstitial and alveolar edema, neutrophil and macrophage cellular infiltration, and hemorrhage. These effects of endotoxin in the lung are dose-dependent. See, for example, Neely et al. (1997) Am J Physiol. Lung 272:L353-L361 and Meyrick et al. (1986) Am. J Pathol. 122:140-151. The direct effect of endotoxin on pulmonary arterial endothelial cells (PAECs), include contracture and widening of interendothelial junctions, cytotoxic effects such as “ruffling” of the surface, prominent cytoplasmic extensions, and nuclear crenation. Meyrick, supra. These electron micrographic changes in PAECs occur as early as 30 minutes following exposure to endotoxin and are associated with changes in permeability. Id. This direct cytotoxic effect of endotoxin on PAECs is dependent on the dose of endotoxin and the presence of serum and is independent of the presence of neutrophils and macrophages. See, for example, Meyrick, supra; Brigham et al. (1987) J Appl. Physiol. 63(2):840-850; Maeda et al. (1995) Shock 3:46-50; and Meyrick et al. (1989) J Cellular Physiol. 138:165-174. In addition, endotoxin-induced cytotoxicity of PAECs is associated with the release of oxygen free radicals and by products of lipid peroxidation, e.g. TXA2. See Brigham, supra and Conary et al. ( 1994) J Clin. Invest. 93:1834-1840.
Previously, it was reported that A1 adenosine receptor (AR) antagonists block endotoxin-induced acute lung injury (ALI) in animals. See Neely et al. (1997) Am J Physiol. Lung 272:L353-L361. In spontaneously breathing, intact-chest cats, in an isolated perfused left lower lobe of the lung, under conditions of controlled blood flow and constant left atrial pressure, endotoxin [15 mg/kg, intralobar arterial (i.a) infusion] produced ALI. Endotoxin (15 mg/kg, i.a.)-induced alveolar injury was blocked in a highly significant manner by A1 adenosine receptor antagonists, 1,3-dipropyl-8-cyclopentylxanthine (DPCPX) and bamiphylline. Furthermore, it has also been reported that endotoxin binds to and activates A1 adenosine receptors on human PAECs to induce the release of TXA2 and IL-6. Wilson et al. (2002) J Endotoxin Res. 8:263-271. Both TXA2 and IL-6 are cytotoxic to endothelial cells. See Zamora et al. (1993) J Appl Physiol 74:224-229 and Gornikiewicz et al. (2000) FASEB J 14:1093-1100. These data suggest that by activating A1 adenosine receptors on PAECs, endotoxin produces cytotoxicity of PAECs that ultimately leads to disruption of the blood-air barrier, interstitial and alveolar edema, neutrophil and macrophage cellular infiltration, as well as hemorrhage in endotoxin-induced ALI. Thus, A1 adenosine receptor antagonists may be useful in minimizing or preventing endotoxin-induced organ (e.g., lung) damage associated with sepsis.
Today, therapy for sepsis includes antibiotics, surgical drainage of the site of suspected infection, inotropes and vasopressors to support the heart and blood pressure, and supportive care in an intensive care unit, including mechanical ventilation. Despite the availability of newer antimicrobial agents and improved supportive care, the mortality for severe sepsis and septic shock of 30-60% remains high, and the outcome remains poor for patients with septicemia. These discouraging statistics suggest a need for additional therapies (adjuvant therapies) to this conventional approach to sepsis, which interrupt the complex cascade of events leading to shock, multi-system organ dysfunction, and death. Adjuvant anti-sepsis therapies would ideally prevent organ damage, shock, and death and improve outcome without interfering with the normal host innate immune response to bacterial infection.
A number of pharmaceutical companies have tried and failed to develop adjuvant anti-sepsis therapies. These companies have tried to develop drugs with anti-endotoxin, anti-cytokine, and other therapeutic properties for the treatment of sepsis. These attempts have failed to demonstrate a beneficial effect in patient outcome. Therefore, a need exists for more effective methods and compositions for treating sepsis.